Frost and Condensation on Aluminum Window Frames

In many condos and older buildings, the first thing to get wet on a cold morning isn’t the glass. It’s the metal frame around it, sometimes with a line of frost along the sill.

Why metal frames go first

• Aluminum conducts heat extremely well, roughly 160 W/m·K by one manufacturer’s figure, versus about 0.3 for fiberglass. A frame without a thermal break pipes outdoor cold straight to its indoor face.
• When that face drops below the room air’s dew point, water condenses. Below 0 °C it turns to frost. Fleetwood’s condensation note says frost is common on highly conductive metal frames and rare on thermally broken ones.

What a thermal break does and doesn’t do

A thermal break is an insulating strip, usually polyamide or poured polyurethane, that splits the frame into inner and outer parts. It raises the inner surface temperature, but it reduces condensation rather than eliminating it. Screws, corner keys, hardware, and metal sill pans can bridge the break. Glazing (double vs triple), spacer type, indoor humidity, and whether room air reaches the window all matter too. A study in the ASCE Journal of Architectural Engineering found that condensation depends on how the frame, glass, and spacer each perform, not just on a window’s overall U-factor or condensation-resistance rating.

What helps in an existing home

1. Lower indoor humidity on cold nights. Metal frames set a lower ceiling than wood or vinyl, so aim for the low end of the outdoor-temperature humidity chart.
2. Let warm air reach the window. Deep reveals, closed blinds, and heavy drapes trap a cold pocket against the frame. Open them at night, or leave a gap.
3. Don’t block heat sources under the window with furniture.
4. Wipe and dry sills each morning so meltwater doesn’t soak into drywall or trim.
5. When replacing, look for a wide thermal break, warm-edge spacers, and a published condensation rating. Ask about frames made from less conductive materials.

Sources: Fleetwood, Condensation on High-Performance Glass; Windows Guy, Thermally Broken Aluminum Windows Explained; F1 Composite (fiberglass frame maker; conductivity figures); ASCE Journal of Architectural Engineering, Condensation Study.

Originally published at https://dry-glass-notes.surge.sh/articles/aluminum-window-frame-condensation-frost.html (AI-assisted, sources cited). If this helped, zaps go to hollisvarrow3753@coinos.io.